I wrote the post at the end a while back, and today a solved the problem.
I added the noperm option to my mount line, good or not, now it works.
Excerpt from man mount.cifs:
Client does not do permission checks. This can expose files on this
mount to access by other users on the local client system. It is
typically only needed when the server supports the CIFS Unix Exten‐
sions but the UIDs/GIDs on the client and server system do not match
closely enough to allow access by the user doing the mount. Note
that this does not affect the normal ACL check on the target machine
done by the server software (of the server ACL against the user name
provided at mount time).